Cream the butter and sugar in a bowl until pale and fluffy.
Stir in eggs and vanilla.
Add cocoa powder, fold in well. Add flour and baking soda. Fold until well combined. Add milk if required and fold in to achieve the desired consistency. The batter should be easy to level with a little shake.
Pour into a lined 20cm cake tin (a larger tin will yield a shallower cake). Shake gently to level. Bake in a preheated oven at 160C for 40-50 minutes or until springy to touch. A skewer entered into the middle of the cake should come out clean once cooked through.
Cook before removing from tin. Then cool completely before icing.
